well it's not a definite universe ender, i mean that's the whole definition of a paradox - a special case where several applicable rules conflict with each other. it might kill everyone, it might not.
You go back in time to kill Riddler.
The future has been altered which means you never went back in time to kill Riddler since it already happened.
That is already a paradox.
Not to mention the fact that the alternate future could even mea you never existed as your ancestors could have probably stayed in their home country instead of fleeing to america or whatever needs to be done to prevent your birth